NEWS AND VIEWS

Lophophorates prove likewise variable

Evidence from molecular phylogeny challenges the conventional wisdom that places an obscure but pivotal group of organisms close to our own ancestry.

BRACHIOPODS were once called lamp Embryological differences, though, are but excludes arthropods. shells, because they look like the lamps not always as clear-cut as these simple defi­ Second, the position of bryozoans, seen in productions of Aladdin. Like nitions imply. To complicate matters, which are lophophorates and yet primitive Aladdin's magic lamp, brachiopods lophophorates exhibit features that suggest with respect to annelids, molluscs and the have a genie inside. It is a feeding structure a protostome affinity. Brachiopods, for other lophophorates, suggests that called a lophophore, a horseshoe-shaped example, have chitinous, hair-like struc­ annelids and molluscs had a lophophorate arrangement of ciliated tentacles surround­ tures similar to the setae of annelids. ancestry, but their affinities have been ing the mouth. This organ marks bra­ If conventional wisdom has it that masked by the secondary loss of this defin­ chiopods as different from bivalve deuterostomes represent a monophyletic itive structure. molluscs, which they resemble only superfi­ pearl amid a paraphyletic, protostome Critics can hardly balk at the suggestion cially. But lophophores are also found oyster, then the lophophorates can be that annelids and molluscs are lophophor­ in bryozoans, and marine worms called placed as the nacre closest to the pearl, and ates that happen to have lost their phoronids. Tradition has grouped these yet not of it. Although protostomes, they lophophores, for the traditional alignment three phyla together on the basis of occupy a special place as those closest to of lophophorates with deuterostomes the shared presence of the lophophore, the ancestry of deuterostomes, having requires that chordates and echinoderms on the grounds that such a complex organ acquired features associated with deuteros­ should have suffered the same loss. is unlikely to have evolved more than once. tome ontogeny, but not yet having lost Nevertheless, one is entitled to wonder Fitting the lophophorates into the larger primitive traits of protostomes. whether Halanych et al. are justified in scheme of things is more difficult. And so it would stand, were it not for throwing out much ontogenetic and mor­ Animals more complex than flatworms the unquiet stirrings of molecular evi­ phological evidence on the basis of the conventionally belong to one of two large dence. Phylogenetic reconstruction based sequence of a single molecule: particularly groups, the protostomes or the deuteros­ on ribosomal RNA gene sequences has as there is another group of lophophore­ tomes. Like Montagues and Capulets, the allied brachiopods with molluscs. But the bearing animals, the pterobranchs, which differences between the two are ontogenet­ evidence gathered hitherto has been do not feature in their analysis. ic and hard to tell from adult appearance. tentative and easily dismissed as more Like the lophophorates, pterobranchs In protostomes, the hole in the end of the protosome primitiveness. are small, obscure sea creatures. Only blastula becomes the mouth, cleavage Perhaps no longer. Complete ISS ribo­ three genera are known, but all have between the cells of the early embryo tends somal RNA genes are now available for lophophores derived from the second pair to be spiral, development is determinate, many species. Kenneth M. Halanych and of coelomic cavities. Yet zoologists do not and the coelom (or body cavity) originates colleagues use them to show that group pterobranchs with lophophorates, by the splitting of pre-existing mesodermal lophophorates are grouped firmly within but with the acorn-worms or enterop­ layers. Prominent protostomes include the protostomes (Science 267, 1641-1643; neusts, which lack lophophores. Ptero­ molluscs, arthropods and annelids: 1995). Several different methods of recon­ branchs and enteropneusts together Drosophila is a protostome. struction reach essentially the same result. comprise the hemichordates, and are In deuterostomes, in contrast, the hole Lophophorates are not just protostomes deuterostomes. If the lophophorates are in the end of the blastula becomes the anus but advanced protostomes, and have no protostomes, as Halanych et al. suggest, and the mouth develops separately special relationship with deuterostomes. then the lophophore must have evolved (whence 'deuterostome', or 'second In the consensus tree presented by twice independently. The extent to which mouth'), cleavage is radial, development Halanych et al., protostomes emerge as a molecular evidence from hemichordates indeterminate, and the coelom develops at strongly supported clade, but the deuteros­ would dent the monophyly of the new­ the same time as the mesoderm, from tomes (in this analysis a crinoid, an made Lophotrochozoa remains to be seen. infolding regions of the primitive gut. The amphioxus and a lamprey) group together And there is one further piece of evidence coelom in many deuterostomes is tripar­ far less well. Within the protostome group, to be considered. It comes (one will no tite, arranged as three cavities from front to the arthropods branch off first and (now, longer be surprised to learn) from yet back. The second and third coeloms, if not this is the interesting part) the bryozoans another group of obscure sea creatures, always the first, are in turn subdivided into come off next. this time one with which even zoologists left and right halves. Deuterostomes This leaves a 'crown' group of are unlikely to have made much acquain­ include echinoderms and chordates: the brachiopods, phoronids, molluscs and tance. From the abyssal depths of the best known is Homo sapiens. annelids. Within this crown, no species is bathyal ooze has emerged a variety of Cleavage in lophophorates is generally closer to any other, except that phoronids acorn-worm of unusually piebald charac­ thought to be radial, and the coelom is usu­ emerge as closer to articulate bra­ ter. Although they look like the acorn­ ally held to evolve from infoldings from the chiopods (in which the shells are hinged worms from inshore waters, they sport gut. The coelom also tends to be tripartite together) than inarticulate ones (in which lophophore-like fringes of tentacles grow­ (although the presence of the foremost they are not). ing from a region that corresponds with the compartment is debatable), and the two In general, the result has fascinating second pair of coelomic compartments. halves of the lophophore originate from implications. First, the presence of These animals are almost certainly the two halves of the second pair of the lophophore defines a subgroup of the deuterostomes, yet a lophophorate by any coelomic cavities. It is along these lines that protostomes (which the authors call other name would smell as sweet. Whether lophophorates have been allied with the the Lophotrochozoa) that includes the same applies to the Lophotrochozoa is deuterostomes. the lophophorates, molluscs and annelids, less certain. Henry Gee NATURE . VOL 374 . 6 APRIL 1995 493